Radiometric Dating - Types
: (Back to Radiometric Dating Home) '' '' : (Or Back to Radiometric Dating Overview) While the overall method of Radiometric Dating is the same through each type, the specific isotopes that are used changes. With these changes in materials, different types of dating apply to different situations. Depending on what is being dated (what it is composed of, where it resides) some methods will be more effective than others. On this page, you will find descriptions of several of the most popular and useful methods that exist today. Radiocarbon Dating By far, the most well-known type of radiometric dating is method using the radioactive isotope of carbon, carbon-14. Every living organism on the planet contains the element carbon. Carbon's most abundant and stable isotope has a mass of 12 (six protons and six neutrons). However, there is also a neutron-rich radioactive isotope of carbon. Carbon-14 has two more neutrons than stable carbon, and thus has a mass that is greater by two. Carbon-14 is produced by a chemical reaction between stable Nitrogen atoms or ions and free neutrons in the atmosphere. Carbon-14 is present a level of about 1 part per trillion in the atmosphere--for every trillion particles of carbon one is C-14. While this is an extremely trace amount, over time it is incorporated into the systems of most life forms. Since C-14 is formed in the atmosphere, it is most abundant in CO2 in the atmosphere. This is how it is incorporated into life forms, both plants and animals readily incorporate atmospheric gases into their systems. This lays the framework for radiocarbon dating. Carbon-14 is a radioactive element, it spontaneously undergoes beta decay and forms Nitrogen-14, a stable isotope of a different element (one less proton). When a life form is alive and undergoing some sort of respiration, it will be gaining C-14 at a relatively high rate (it will equilibrate with the C-14 in the atmosphere) and losing it to decay at a different rate. However, when the life form dies and is buried or otherwise stops interacting with atmospheric gases there will be no further increase in the amount of radioactive carbon in the system. Therefore, the amount of radioactive carbon in the system will only decrease and form Nitrogen. Through experimentation it has been determined that Carbon-14 has a half-life of about 5,370 years. This means that if the amounts of C-14 and N-14 in a sample are known, then its age can be calculated for a wide range of years. Several half-lives can pass and still leave measurable amounts radioactive carbon present in the sample. Radiocarbon dating is so well-known because it can be applied to nearly any organism that was living at some time. http://www.nosams.whoi.edu/about/carbon_dating.html Potassium-Argon Dating While radiocarbon dating is very useful in cases where organic matter is being analyzed, in many cases the samples that are being dated were never alive. For instance, ancient tools are often pieces of stone that have been fashioned into different shapes; determining the age of specific tools has strong implications in the field of archaeology. In some cases, an organic sample is indeed being analyzed, but for some reason it has been contaminated by radioactive carbon or is undatable by radiocarbon methods for some other reason. In this case determining the age of the surrounding earth or rock materials can be very helpful in determining the age of the sample. Potassium-Argon (K/Ar) dating is a method that applies directly the dating of rocks. It is especially useful in determining the age of volcanic rocks which often contain potassium. Like carbon, potassium has three natural isotopes, one of which is radioactive. One of the primary decay products of K-40 is Ar-40--a stable and unreactive gas. Potassium decay has been established as the only source of Argon in volcanic rocks; it is a gas so most of it escapes unless it is formed after the molten rock has solidified, and it is not a major product in any other reactions. The half-life of potassium-40 has been measured to be about 1.3 billion years. This method makes it possible to determine the relative age of samples, it is also useful in approximating the ages of items in a small area by establishing the date of a portion of the rock in that area. http://www.amazon.com/World-Beginnings-4000-Oxford-History/dp/0195167120 Uranium-Lead Dating Uranium-Lead (U/Pb) dating is one of the oldest methods of radiometric dating, Rutherford being one of the first scientists to note it. Like K/Ar dating, U/Pb dating is more suited to determining the age of matter that was not ever alive. It is especially disposed to dating minerals such as Zircon. Zircon is a fairly abundant mineral that exists in many forms in the environment, and because of its chemical structure Uranium is easily incorporated into its molecules. However, Lead is not easily incorporated into the Zircon molecules; therefore, when a sample of Zircon is analyzed any lead that is found must have come directly from Uranium decay. There are two isotopes of Uranium, both of which are radioactive. U-238 decays to Pb-206, and U-235 decays to Pb-207. These two decay pathways have different half-lives that have been measured and recorded. These factors contribute to create a very reliable dating method. Based on the ratios between parent and daughter atoms in samples, a single age is usually indicated by both decay pathways. This lead to the development of the Concordia Diagram, this is a diagram mapping the correlation between the ratios to assert a certain age in a sample. U/Pb dating is an excellent method in mineral dating because multiple factors contribute to the accuracy of the method. http://geology.about.com/od/geotime_dating/a/uraniumlead.htm References #NOSAMS: What is carbon dating?. (n.d.). Retrieved from http://www.nosams.whoi.edu/about/carbon_dating.html #Tattersall, I. (2008). The World from Beginnings to 4000 BCE. New York, New York: Oxford University Press. #Alden, A. About uranium-lead dating. (n.d.). Retrieved from http://geology.about.com/od/geotime_dating/a/uraniumlead.htm